1. Field of the Invention
The present invention relates to a thin film transistor using an organic semiconductor and a method of manufacturing the thin film transistor, and more particularly to a transistor having a structure in which an organic semiconductor that forms a channel is made of a single crystal or a polycrystal of organic molecules, and having a structure in which the orientation of the single crystal is directed to a specific direction with respect to a direction that connects electrodes on both ends of the channel to each other, or having a structure in which grains that constitute the polycrystal are arranged in a specific direction with respect to a direction that connects the electrodes on both ends of the channel to each other, and a method of manufacturing the transistor.
2. Description of the Related Art
In a flat-screen display device using a liquid crystal or organic electro luminescence element, there is used a thin film transistor (TFT) using an amorphous silicon or a polycrystalline silicon for a channel as an element that drives a pixel. On the other hand, in the display device using the organic EL for the pixel, study has been widely conducted in which a TFT which is used in a driver circuit is intended to be also made of an organic material in order to realize a flexible display device or reduce the manufacturing costs. Because the TFT that is made of amorphous silicon or polycrystal silicon is not flexible, it is impossible to make the display device using those TFTs flexible. Also, the manufacturing costs become high because vacuum equipments are used in the manufacturing process. If it is possible that the TFT can be made of an organic material, the flexible display device can be realized. Also, the display device can be manufactured through a so-called wet process such as a printing technology depending on an organic material to be used, and no vacuum equipment is required for manufacturing, and therefore a reduction in the manufacturing costs can be also realized.
The organic molecules are roughly classified into organic molecules that are low in the molecular weight (low-molecular-weight organic molecules) such as a monomer or an oligomer, and organic molecules that are high in the molecular weight (high-molecular-weight organic molecules) such as a polymer. The TFT (organic TFT) having a channel made of the organic molecules can be also roughly classified into two kinds of TFTs according to the type of molecules of which the channel is made up. It is proved that organic TFT that uses the low-molecular-weight organic molecules for the channel is capable of increasing the mobility of carriers that flow in the channel to the same degree as that of the amorphous silicon when organic TFT is capable of excellently keeping the crystallinity of the organic molecules, and such an organic TFT has the feature that a TFT which is high in the operating speed as the organic TFT is readily obtained. However, because vacuum deposition of the organic molecule is generally employed in the formation of the channel, there is an advantage in that it is difficult to reduce the manufacture costs. On the other hand, the organic TFT that uses the high-molecular-weight organic molecules as the channel is capable of reducing the manufacture costs because it is easy to apply the wet process to the manufacture. However, the mobility of the carriers that flow in the channel can only be realized about 1/10 the amount of that of the organic TFT that uses the low-molecular-weight organic molecules for the channel, and there is a disadvantage in that the performance of the TFT is low.
In general, in the case of using the organic molecules for the channel, there arises such a problem that the operation speed of the TFT is lower than that of the TFT of silicon. This problem is caused by the fact that the mobility of the carriers that flow in the channel is small, and there has been known that one of the large causes is the scattering of the carriers within the channel. In order to reduce the scattering of the carriers, there has been widely conducted that, in the case of the low-molecular-weight organic molecules, the size of grains of crystals of forming the channel are increased and the number of grain boundaries are reduced, through which the carries pass through when the carriers are conducted between electrodes at both ends of the channel. To use the single crystal for the channel is most desirable because the influence of the grain boundaries can be removed. In the case of high-molecular-weight organic molecules, it is general that the high-molecular-weight organic molecules are extended in parallel with the conduction direction of the carriers that flow in the channel as much as possible, to thereby reduce the scattering of the carries within the high-molecular-weight organic molecules.
In order to realize the flexible display device, it is necessary to provide the flexibility including a peripheral circuit that drives the pixels. The carrier mobility of about 1 cm2/V·s or more is required for the TFT that is used in the circuit which drives the pixel, but the organic TFT that has been proved to satisfy the above requirement under the circumstances is only the TFT that uses the low-molecular-weight organic molecules for the channel. For example, in Science, Vol. 303, page 1644 (2004), the carrier mobility of 15 (cm2/V·s) is obtained in the organic TFT that uses single crystal of rubrene molecules for the channel. Also, in Applied Physics Letters, Vol. 84, page 3061 (2004), there has been reported the carrier mobility of 35 (cm2/Vs) at a room temperature with respect to the single crystal of pentacene molecules which have been highly purified. However, the above high mobility is applied to the single crystal specimen, and obtained by paying specific attention to the manufacture of the single crystal and the TFT specimen. In the case where the low-molecular-weight organic molecules are used for the channel, it is more general to use thin-film crystal of the organic molecules that have been produced by vacuum deposition. However, this method makes it difficult to form the single crystal for the channel; and is disadvantageous in the costs or the mass productivity because vacuum is applied.
The above organic TFT suffers from such a problem that the TFT cannot perform both of the performance that is required from the application side and the costs and productivity which are required from the viewpoint of the productivity. In other words, it is general that the organic TFT which consists of low-molecular-weight organic molecules which is liable to enhance the performance of the TFT is manufactured by the aid of vacuum deposition, and therefore disadvantageous in the viewpoint of manufacture. On the other hand, the organic TFT that is made of the high-molecular-weight organic molecules which is liable to suppress the manufacture costs is remarkably low in the performance of the TFT, and can be therefore applied to nothing other than the limited intended purposes.
As means for solving the above problem, there is a method of dissolving the low-molecular-weight organic molecules in a solvent and coating the solution to form a semiconductor layer of the channel. As to pentacene that is most typical organic molecules as an applied example of the low-molecular-weight organic molecules to the TFT, for example, Journal of Applied Physics, Vol. 79, page 2136 (1996), and Journal of American Chemical Society, Vol. 124, page 8812 (2002) have reported a technique by which precursors of pentacene molecules are synthesized, and a thin film is formed by using a solution that increases the solubility with respect to the solvent. Also, Synthetic Metals, Vol. 153, page 1 (2005) discloses a technique by which the pentacene molecules are directly solved in the solvent, and coated to form a thin film. In addition, Applied Physics letters, Vol. 84, page 3061 (2004) and Japanese Journal of Applied Physics, Vol. 43, No. 2B, page L315 (2004) disclose a procedure of solving pentacene molecules in the organic solvent.
Those techniques make it possible to form the low-molecular-weight organic molecules by coating by using no vacuum apparatus, and possibilities of realizing the required performance at the low costs may be opened.